There has been an approach to decrease rolling resistance of a tire to suppress the heat build-up and thus improve fuel economy of a vehicle. The demand for improving fuel economy by means of tire improvement has increased in recent years. In addition to treads and sidewalls, other tire components such as clinch apexes are also required to be improved to contribute to better fuel economy (lower heat build-up).
Examples of known methods for reducing the heat build-up of rubber compositions for a clinch apex or the like include a method using semi-reinforcing filler, a method using a smaller amount of filler, and a method using silica as filler. However, such methods cause reduction in the effect of reinforcing the rubber composition, and thereby problematically cause reduction in breaking properties. Thus, it has been difficult to achieve both better fuel economy and higher breaking properties at the same time.
Meanwhile, clinch apexes are commonly produced using natural rubber. Here, natural rubber has a higher Mooney viscosity than synthetic rubbers and thus has low processability. Therefore, natural rubber to be used is usually mixed with a peptizer and masticated so that the rubber has a reduced Mooney viscosity. In the case of using natural rubber, requirement of such a process decreases productivity. Further, the mastication causes molecular chain scission in natural rubber, thereby problematically leading to a loss of the properties of a high-molecular-weight polymer that natural rubber essentially has (for example, fuel economy and rubber strength).
Patent Literature 1 discloses a rubber composition prepared using a natural rubber that has been deproteinized to reduce the total nitrogen content to 0.1% by weight or less. However, the rubber composition still has room for improving fuel economy, flex crack growth resistance, and processability in a balanced manner.